Seismic Swarm PS20100227.1: Analysis of the February 2010 Event off Constitución, Chile

The seismic swarm designated PS20100227.1 occurred on 27 February 2010, approximately 126 km west-northwest of Constitución, Chile. The sequence began at 06:52 and concluded at 22:20, encompassing 22 earthquakes within a span of 15 hours and 27 minutes. All events were recorded at focal depths between 20 km and 38 km, with the majority clustered near 35 km. Magnitudes ranged from 5.0 to a peak of 6.2, reflecting moderate but clustered seismic energy release typical of swarm activity in subduction settings.

This swarm unfolded within the tectonically active Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6–7 cm per year. The region exhibits persistent seismicity due to frictional locking along the megathrust interface and associated intraslab deformation. Depths around 35 km align with the downdip extent of the seismogenic zone in this segment of the margin, where brittle failure occurs in the oceanic crust and overlying wedge.

The temporal progression showed an initial cluster of larger events in the morning hours, including the 6.2 mainshock at 06:52:34 followed closely by multiple 5.0–5.2 aftershocks. Activity intensified again around 08:25 with a 6.1 event, then transitioned to a more diffuse pattern of 5.0–5.3 shocks throughout the afternoon and evening. Later events exhibited slightly shallower depths of 20–26 km, suggesting possible migration toward the upper plate or along-strike adjustment within the fault network. Such patterns indicate fluid migration or stress redistribution along pre-existing fractures rather than a single mainshock-aftershock cascade.

Geological context for the area includes the broader Maule segment of the subduction zone, which has hosted repeated great earthquakes. Historical records document major ruptures in 1835 and 1906, with recurrence intervals estimated at 100–150 years based on paleoseismic and geodetic data. The 2010 swarm occurred in a portion of the margin that had accumulated significant strain since the previous century-scale event.

Since 2000, only one swarm has been documented in the immediate vicinity prior to this sequence, underscoring the relative rarity of such clustered moderate-magnitude activity. A subsequent strong earthquake of magnitude 7.1 occurred on 25 March 2012, located 23 km northeast of Constitución and roughly 43 km from the swarm centroid, illustrating ongoing post-2010 stress adjustments along the margin.

Seismic swarms in this environment provide valuable indicators of transient stress changes that may precede or accompany larger ruptures. Monitoring of depth and magnitude distributions, as observed here, aids in distinguishing swarm behavior from typical aftershock decay and supports refined hazard assessments for coastal communities in central Chile.

References

SeismoSight internal swarm classification records, PS20100227.1 dataset.
USGS Earthquake Catalog, regional subduction zone parameters.
Global CMT Project, focal mechanism and depth constraints for Chilean margin events.